This invention relates to apparatus for determining the crystalline quality of material of a semiconductor surface using light.
In manufacturing semiconductor devices, the surface of the semiconductor material in which the devices are fabricated must be substantially free of both physical and crystalline defects. A high degree of crystalline perfection is necessary to produce reliable devices having good electrical properties. In order to control the properties of such devices, it is necessary to be able to determine the quality of semiconductor material that is being used to make the devices.
The light reflectance of the surface of a semiconductor is generally dependent on the physical and crystallographic condition of the surface.
Physical surface damage such as scratches, pits and surface roughness, resulting from lapping and polishing procedures, can be detected by light scattering effects in the visible region of the spectrum, for example, by the use of conventional lasers to scan the semiconductor surface. Similarly, the surface texture of homoepitaxial and heteroepitaxial films, prepared on various substrates by chemical vapor deposition (CVD), can also be detected in similar fashion. An example of particular importance is silicon-on-sapphire (SOS). Surface texture of "haze", observed visually because of light scattering, has been attributed in the past to inferior quality SOS material.
Crystallographic damage (or what may be also termed "lattice disorder"), which may also result from lapping and polishing procedures or may be present in as-grown homoepitaxial or heteroepitaxial semiconductor films such as SOS, is not easily detected by reflectance methods using visible light. This is due to the fact that the well known "optical" constants of a semiconductor such as crystalline silicon are not appreciably influenced by lattice disorder in the visible region of the spectrum. However, the optical constants of silicon are significantly influenced by lattice disorder at photon energies near 4.3 eV which corresponds to the well known X.sub.4 -X.sub.1 silicon transition. This transition occurs at a wavelength of about 2880 angstroms in the ultraviolet region of the spectrum. Thus, the light reflectance of silicon, which is a function of the optical constants, is sensitive to crystalline damage or lattice disorder in the UV region of the spectrum. Other semiconductors display similar reflectance properties at their corresponding characteristic wavelengths.
In copending patent application Ser. No. 189,348 entitled "A Method and Apparatus for Determining the Quality of a Semiconductor Surface," filed on Sept. 22, 1980 by M. T. Duffy and P. J. Zanzucchi, there is described a method for determining the quality of the material of a semiconductor surface. In brief, the surface quality of the semiconductor material, as described therein, is determined by exposing the semiconductor surface to two light beams of different wavelengths or wavelength ranges (e.g. ultraviolet at 2800 angstroms and near ultraviolet at 4000 angstroms). A portion of each of the respective light beams is reflected from the semiconductor surface. The intensity of each reflected beam is measured to obtain an intensity difference whereby the magnitude of the difference is a measure of the quality of the semiconductor material.
There is a need, however, for inexpensive apparatus that is capable of performing rapidly and accurately, the steps, of what is conveniently termed the "two wavelength reflectance measurement method," described in that copending application.